Authors: Defined at, and elevated to, Group level in this Lexicon. It largely equates to the Andam Formation as described by Oprinsen (1986), Hughes Clarke (1988 - based on subsurface data) and revised by Droste (1997). However, the Andam Group, as defined here, also includes the Ghudun Formation. Previously the Andam was a formation, which together with the Ghudun Formation was placed within the Mahatta Humaid Group, as detailed in Droste (1997).

Introduction

Following the continental clastics of the Mahatta Humaid Group, clastic sedimentation continued with marine, marginal marine and marine-influenced deposition in North Oman, represented by the Andam Group. Clear marine indicators include glauconite, bioclasts, palynomorphs and ichnofaunas in cuttings, core and outcrop. The marine influence in the Andam Group generally diminishes towards the south and the Group becomes more proximal shallow marine and coastal/deltaic in character. The Ghudun Formation exhibits the least marine influence of the five formations in this Group. The main reservoir development within the Andam Group is associated with the Barik Formation (Droste, 1997; Aitken et al., 2008; Millson et al., 2008).

Lithology: The Andam Group comprises a dominantly clastic succession of sandstones, siltstones and shales. Sandstones are typically fine- to, more rarely, medium-grained and the shales are red, reddish-brown and greenish-grey in colour. Limestone (commonly dolomitic) only really occurs in the Al Bashair Formation, although Droste (1997) also notes the occurrence of dolomite at the top of the Mabrouk Formation.

Subsurface recognition: Whilst drilling, the Andam Group is difficult to identify on lithology alone as the lithologies of all formations of the Andam Group and of the underlying Miqrat Formation (Mahatta Humaid Group) are similar. The Al Bashair Formation is the only one with limestone interbeds. The grain size of the Andam sandstones (generally fine- or medium-grained) is similar to those of the underand overlying formations (from the Mahatta Humaid and Safiq groups) with the exception of the Al Khlata sandstones which are generally coarser grained. Shale colour may be the best feature to mark the change from the Safiq (predominantly grey with minor quantities of reddish/orange brown) or Al Khlata (predominantly dark grey) to the upper Andam Group (Ghudun Formation – mottled red and green shales). However, there are no lithological differences between the red shales of the Safiq Group and the red shales of the Andam Group.

Details of subsurface recognition are listed under the respective formations.

The Barakat, Mabrouk, Barik and Al Bashair formations cannot be differentiated in South Oman as they were either not deposited or have been eroded beneath younger formations. Typically, all deposits between the Mahwis and the Al Khlata (or Misfar, where preserved) are assigned to the Ghudun Formation. It is certain that the Ghudun Formation occurs in South Oman, especially in the western part of the South Oman Salt Basin. It is possible that remnants of the lower formations of the Andam Group occur in the Eastern Flank of South Oman, but these may be older. This uncertainty is exacerbated by a lack of biostratigraphic control.

Boundaries:Droste (1997) indicated that the base Andam represents a regional unconformity based on subtle seismic onlapping geometries and truncation of the underlying Miqrat and Amin formations in the Fahud Salt Basin. However, the base Andam Group is transgressive with a relatively subdued ravinement surface. Locally, this may be reflected by an angular unconformity that is related to penecontemporaneous halokinesis. The base of the Andam Group is marked by a Gamma log break to higher values at the top of the Miqrat Formation. In many wells it corresponds to a thin calcareous sandstone bed overlying the Miqrat Formation. This boundary marks the onset of a series of backstepping, cleaning-upward cycles (parasequences). If the thin, basal, calcareous sandstone cannot be distinguished, the boundary is placed at the base of the first Gamma log break to lower values above the Miqrat Formation.

The top of the Group is a major unconformity overlain by the typically clay-rich beds or sandstones with blocky Gamma log pattern (unlike the highly serrate Ghudun Gamma profile) of the Safiq Group, or the diagnostic glaciogenic sands, grey shales and diamictites of the Al Khlata Formation.

Distribution: The Andam Group occurs extensively across Central and North Oman, the thickest development is in the centre of the Ghaba Salt Basin. Towards the south and west a decrease in thickness occurs due to stratigraphic thinning, internal truncation(s), probably at the bases of the Ghudun and Barakat formations, and onlap onto the Central Oman High. Along the eastern edge, towards the Al Huqf outcrop area, the Andam Group is truncated below the base Haushi unconformity. In South Oman the Group is largely represented by the Ghudun Formation and is mainly confined to the west of the South Oman Salt Basin. However, it is possible that isolated remnants of the pre-Ghudun Andam Group also occur on the Eastern Flank. The Andam Group may be tentatively correlated with the surface Amdeh Formation units 2, 3 and 4 (Le Métour et al., 1986; Rickards et al., in preparation) in the Saih Hatat area of the Al Hajar Mountains, but dating to fully support this correlation does not exist.

Deposition: The Andam Group comprises sediments deposited in shallow marine to coastal plain and braid delta settings, as detailed at Formation level. It is important to stress that continental depositional settings and sediment composition was not influenced at all by any land plants.

Subdivision: The Andam Group is subdivided into five formations: Ghudun, Barakat, Mabrouk, Barik, and Al Bashair.

Age: ‘Late’1 Cambrian – early Middle Ordovician, Furongian – early Dapingian, ca. 499–470 Ma. The maximum flooding surfaces of Sharland et al. (2001) have been positioned as noted above. See their figure 4.9 (well Saih Nihayda-33) where they place Cm30, O10 and O20 in the Al Bashair, Mabrouk and Barakat formations, respectively.

Biostratigraphy: Palynological data is the foundation for subdivision within the Andam Group (Mohiuddin et al., 2007 and Booth, 2009). Two Palynozones and three Palyno-subzones are recognised as detailed below (see also figure 12.1).

Zone

Subzone

Marker species

Relative age

Formation/Unit

1100

Coryphidium bohemicum

late Floian – early Dapingian

Upper Ghudun

1108

Acanthodiacrodium spp.

Furongian – Tremadocian

Al Bashair – Barakat

C

Acanthodiacrodium spp.

(late Tremadocian)

(Barakat)

B

Vulcanisphaera africana

(Tremadocian)

(Mabrouk – Barakat)

A

Veryhachium? dumontii

(Furongian)

(Al Bashair)

Zone

Subzone

Marker species

Relative age

Formation/Unit

1100

Coryphidium bohemicum

late Floian – early Dapingian

Upper Ghudun

1108

Acanthodiacrodium spp.

Furongian – Tremadocian

Al Bashair – Barakat

C

Acanthodiacrodium spp.

(late Tremadocian)

(Barakat)

B

Vulcanisphaera africana

(Tremadocian)

(Mabrouk – Barakat)

A

Veryhachium? dumontii

(Furongian)

(Al Bashair)

Significant portions of the Andam Group are barren of palynomorphs, as shown in Figure 12.1, e.g. to date there is no proven recovery from the Barik Formation.

Fortey (1994, 1995) describes fragments of the late Furongian trilobite Eosaukia cf. walcotti from flat bedded sandstones, near the base of the Barik Formation in the Al Huqf outcrop (Droste, 1997). Other trilobite assemblages, which include Paracoosia cf. mirzadi and Changshania kürsteni were recovered from the nearby Al Bashair Formation outcrops. These suggest an equivalent early to middle Furongian age (Fortey, 1994, 1995), and therefore support the palynological evidence.

Ghudun Formation

Introduction

The Ghudun Formation is generally a very thick package of micaceous quartz sandstones with intercalated micaceous siltstones and shales. The unit corresponds to a regression during which a prograding braided coastal plain to braid delta system was deposited.

Lithology: The Ghudun Formation is characterised by variably micaceous quartzose sandstones and intercalated micaceous siltstones and shales, with generally low clay content. Grey and red sandstones are generally fine- to medium-grained, more medium- to coarse-grained in South Oman, and commonly contain greenish-grey mudrock rip-up clasts. Grey shales are absent from the Ghudun, shales are highly micaceous mottled red or green (Figures 13.7 and 13.8). Glauconite, pyrite and bioturbation occur locally, towards the base and top of the Formation.

Subsurface recognition: Whilst drilling, the Ghudun Formation is difficult to identify on lithology alone. The grain size of the Ghudun sandstones (generally fine or fine to medium) is similar to that of the under- and overlying formations, e.g. Safiq Group, Barakat-Mabrouk or Mahwis (with the exception of the Al Khlata sandstones, which are generally coarser grained). Shale colour may be the best feature to mark the change from the Safiq (predominantly grey with minor quantities of reddish/orange-brown) or Al Khlata (predominantly dark grey) to the Ghudun (mottled red and green shales), although there are no lithological differences between the red shales of the Safiq Group and the red shales of the Ghudun and the Mahwis formations.

Commonly the base Safiq, whether Hasirah or Saih Nihayda, is sandy and it is very difficult on cuttings to distinguish these from the Ghudun sands (see Dhahaban-2, Figure 13.6).

Post-drilling, the Safiq Group sands show a general blockier Gamma-log character than the more serrate Ghudun sands. The boundary should be picked at the base of the blocky sands (see also Khasfah-1, Figure 13.5).

Biostratigraphy can help identify the Safiq Group sediments, which are usually rich in palynomorphs, indicating a Darriwilian or younger age.

The serrate Gamma-log pattern is very distinctive and typical of all penetrations. It is an expression of high mica content and possibly variations in the uranium content, which result in the shale-like Gamma character of the sands. This is more clearly visible when various spectral Gamma logs are available. Typical barrel and funnel shaped cycles of 10–100 m thickness can be recognised and correlated regionally. Mudrocks are prone to washout and are also correlatable on a regional scale.

Boundaries: The Ghudun Formation generally underlies the Safiq Group or the Al Khlata Formation and less commonly the Kahmah Group. It generally overlies other formations of the Andam Group in North Oman and the Mahwis Formation in South Oman.

The basal boundary is generally conformable and also diachronous in areas where the Barakat facies becomes sandier. As a consequence it may be difficult to pick. It is picked at the base of the low Gamma, sand-dominated section, above the fossiliferous interbedded mudstones and sandstones of the Barakat Formation. Locally the base may be an unconformity over earlier Haima or Huqf units. The upper boundary is unconformable into the generally clay-rich beds or sandstones with blocky Gamma log pattern of the Safiq Group.

Distribution: The Ghudun Formation is widespread across North and Central Oman, but is mainly confined to the west of South Oman. The Formation is thickest in the centre of the Ghaba Salt Basin (up to 1,400 m) and is generally thin in the Fahud Salt Basin. It thins towards the (south) east and is missing on the Eastern Flank and the Central Oman High (or is very thin there) as a result of truncation below the base Haushi or younger unconformities. Part of the Amdeh Formation (Amdeh 4) in the Saih Hatat area of the Al Hajar Mountains (Le Métour et al., 1986; Rickards et al., in preparation) is probably time equivalent to the Ghudun Formation.

Deposition:Droste (1997) interpreted the Ghudun Formation as a series of stacked braid delta lobes separated by marine flooding events within an overall aggrading succession. Core studies suggest that there may be a significant tidal influence (Sutcliffe and Aitken, 2002).

Generally, the setting seems to be more proximal braided coastal plain to braided delta plain compared to the rest of the Andam Group. A more distal, marginal-marine setting is suggested in the northern part of the Ghaba Salt Basin and the Fahud Salt Basin by the overall finer grain sizes and evidence of marine conditions (glauconite, bioturbation, rare sphaeromorph acritarchs). Shallow-marine, bioturbated sandstones in South Oman have been attributed to the Ghudun Formation (Heward, 1990), but it is also possible that these sands may be older than the Ghudun Formation.

Subdivision: The Gamma character is remarkably consistent, throughout North Oman in particular. This consistency holds even where there are considerable differences in thickness, e.g sections are condensed but retain character. This allows the Ghudun Formation, with some help from subtle changes in other wireline logs, to be subdivided into three informal units, the Lower, Middle and Upper Ghudun members (Figures 13.2 and 13.3).

The Lower Ghudun Member is characterised by a dominantly blocky (aggradational, mildly progradational) Gamma response, generally with a narrow negative (sand-type) Neutron-Density separation.

The Middle Ghudun Member generally features a bow-shaped Gamma response. Base Middle Ghudun is taken at the shift to a dirtying-upward response, which ultimately leads to a Gamma maximum. A thin, more blocky, high Gamma unit may be present at the base of the Middle Ghudun. Maximum Gamma response can be associated with a positive (shale-type) Neutron-Density separation and a mild Sonic ‘kick’. This is overlain by a cleaning-upward Gamma profile, with an increase in negative (sand-type) Neutron-Density separation, although this is always a subtle change and variable.

The base Upper Ghudun is taken at an increase in the Gamma log, which is subsequently followed by a cleaning-upwards Gamma response. The Upper Ghudun is generally truncated by erosion beneath the base Safiq Group or base Haushi unconformities.

Age: Early – Middle Ordovician, Floian – early Dapingian, ca. 478.6–470 Ma. The age of the Ghudun Formation is constrained by the occurrence of Tremadocian acritarchs in the underlying Barakat Formation, and an Early – Middle Ordovician (late Floian – early Dapingian) age, based on acritarchs, in its upper part (Booth, 2009). The marine-flooding event occurring at the top of the Ghudun Formation (Gamma maximum in lower part Upper Ghudun, see Figure 13.3) is not attributable to either MFS O20 or MFS O30 of Sharland et al. (2001) as it lies between these periods of marine activity. It is provisionally referred to as MFS O25 (Figure 12.1).

Biostratigraphy: Most of the Ghudun sediments are barren of microfossils as depositional environments were usually not conducive to microfossil preservation, marine-flooding events were relatively rare, and land plants had yet to evolve. However, a few wells to the northwest, which penetrate the Upper Ghudun Member, have yielded acritarchs suggesting the presence of Coryphidium bohemicum, Palynozone 1100 (Booth, 2009).

Introduction

Overlying the offshore marine Mabrouk Formation the Barakat Formation consists of fossiliferous mudstones with sandstone interbeds that are best developed at its base. It contains a significant maximum flooding surface (O20, Sharland et al., 2001) towards its central part and is overlain by the coastal to delta plain, tidally-influenced sandstones of the Ghudun Formation.

Lithology: The Barakat Formation is a succession of highly bioturbated (dominantly Skolithos with less common Planolites and Thalassinoides and various other rarer ichnofaunas) and fossiliferous (Lingula) shales with some highly heterolithic and sandstone interbeds, which are most common at the base and towards the top of the Formation. The shales are red-brown or green-grey (Figure 13.8), bioclastic and locally glauconitic. Sandstone streaks/stringers are common and mudrocks often feature current ripples, wavy laminae and possible hummocky cross-stratification and may also be disrupted by synaeresis cracks, dewatering, load- and injection structures. The sandstones are very fine- to fine-grained, rarely medium-grained, especially at the base of the Formation, but also locally towards the top. They are light grey to brown-grey and contain low-angle/horizontal lamination, with some inclined and wavy laminae, and locally intra-formational mudrock rip-up clasts. Sandier intervals are commonly highly disturbed, displaying injection, dewatering and load structures.

Subsurface recognition: Whilst drilling, penetration of the Barakat Formation is marked by an increase in shale recovery. However, this boundary is commonly missed while drilling, especially where the Barakat has greater sand content. Where the Barakat is fully developed, the increase in shaliness downwards may be followed by an increase in the percentage of sandstone (basal sand, before the more monotonous Mabrouk Formation shales). Additionally, the Barakat Formation has a lower Rate of Penetration than the overlying Ghudun Formation, with negative drill breaks at upper and lower boundaries.

Whilst drilling, the lower boundary is picked based on either an abrupt change from a sandy succession to more mud-prone cuttings (where the basal Barakat sands are present) or is a more subtle change marked by an increase in shale content. This may not be apparent until several metres to tens of metres of Mabrouk have been drilled.

Post-drill or with ‘logging while drilling’ the overlying Ghudun Formation has lower minimum Gamma values. The upper boundary should be picked directly below the sand-dominated, low Gamma sediments of the Ghudun Formation changing from typically cleaning-upwards, fossiliferous interbedded mud- and sandstones of the Barakat Formation. Overall, where the Formation is fully developed (i.e. with basal sandstone dirtying upwards to a maximum and then cleaning upwards), the Formation has a bow-shaped Gamma response with related changes in the Neutron-Density separation and other logs. However, severe washouts can result in anomalous Density-Neutron and other log readings. The Gamma log is not affected. Locally, particularly to the south, the transgression of the base Barakat unconformity appears to have been very rapid and the Formation only displays the cleaning-upwards signature.

A basal sand (generally blocky, low Gamma) locally separates the Barakat from the Mabrouk. This is marked by a clear Gamma break from the higher readings of the shalier Mabrouk Formation. Where the basal sands are absent additional criteria for picking the base Barakat include truncation of the Mabrouk Formation, a Gamma-log break from monotonous, more or less constant, to a cleaning-upward trend, and the possible occurrence of bioclasts at the base of the Barakat Formation.

Boundaries: The Barakat Formation underlies the Ghudun. The top is conformable, but diachronous, switching to unconformable at basin margins and locally. This boundary is placed at the shift from fossiliferous interbedded mud- and sandstones to the sand dominated, lower Gamma Ghudun sands. It is truncated beneath the Al Khlata and younger formations to the north, east and south across the Central Oman High.

It unconformably overlies the Mabrouk Formation, occasionally the Barik Formation and locally the Huqf Supergroup. The boundary is placed at the base of the basal Barakat sands overlying the more monotonous mudstones of the Mabrouk and is clearly seen in a shift of Gamma readings. In the absence of the basal sand this boundary is difficult to pick. Droste (1997) indicates a significant stratigraphic break at this level increasing to the southern, eastern and western margins of the Ghaba Salt Basin with the potential for at least 300–600 m of missing section. Hughes Clarke (1988) originally interpreted this unconformity at the base of the Ghudun Formation, but based on log correlations, indicating subtle, low-angle truncation and thinning of underlying units, Droste (1997) placed the unconformity at the base of the Barakat Formation. Droste (1997) expressed some uncertainty regarding the position of this unconformity suggesting that it may lie in the upper part of the Mabrouk Formation, an issue that still cannot be resolved with currently available data. The base Barakat unconformity is often very difficult to identify in wireline logs and cuttings as it commonly occurs within an overall shaly succession.

Distribution: The Barakat Formation is widespread over Central and North Oman in the Ghaba Salt Basin and the Fahud Salt Basin and extends into Saudi Arabia. It trends southwest to northeast following the Ghaba Salt Basin and its thickest development occurs in the centre of this basin (more than 350 m). The Barakat Formation is thinner in the Fahud Salt Basin. In North Oman, it is truncated beneath the Al Khlata and younger formations to the north, east and south across the Central Oman High. It apparently does not occur in South Oman, either due to non-deposition or erosion, although a locally observed marine bioturbated sandstone overlying the Mahwis Formation could be a Barakat or Mabrouk equivalent.

The basal sandstones have an irregular distribution that may reflect erosional topographic infill during shoreface retreat or erosional remnants as a consequence of ravinement. They are largely restricted to the central parts of the Ghaba Salt Basin. The mudrocks that dominate the middle part of the Formation are extensive across North and Central Oman. The uppermost coarsening-upwards sandier interval is largely restricted to the western parts of both the Ghaba and Fahud Salt basins, with open-marine sediments dominating the northeastern portion of the Ghaba Salt Basin.

Deposition: The Barakat Formation was deposited in a shallow-marine environment based on the occurrence of bioturbation, bioclasts, palynomorphs, glauconite and possible hummocky cross stratification. A major transgressive-regressive trend can be recognised with a maximum flooding surface (O20, Sharland et al., 2001) in the middle of the Formation.

The sandstones at the base were interpreted as transgressive coastal sands by Droste (1997) and may represent, at least in part, regressive (possibly lower) shoreface-offshore transition zone deposits. However, there are indications from the southern part of the Fahud Salt Basin for more open marine conditions. These open marine-deposits are succeeded by a coarsening-upward into shallower sandier sediments deposited in front of the braid delta complex of the Ghudun Formation, probably representing offshore transition zone to possibly shoreface environments.

Subdivision: The log and lithological characteristics described above allow consistent correlation but no formal subdivisions are proposed.

Age: Early Ordovician, ‘late’ Tremadocian, ca. 482–478.6 Ma. There remains considerable uncertainty with regard to this part of the subsurface section and numeric ages in particular should be viewed with caution. Sharland et al. (2001) place their MFS O20 in the middle of the Barakat, higher Gamma shales.

Biostratigraphy: Well-preserved acritarchs can be recovered from the higher Gamma interval of the Barakat Formation, which indicate the presence of the 1108C (Acanthodiacrodium spp.) and 1108B (Vulcanisphaera africana) Palyno-subzones, implying a Tremadocian age. Support for this interpretation is provided by the report of Early Ordovician conodonts from the top of the Barakat Formation, in Droste (1997).

Lithology: The Mabrouk Formation is dominated by micaceous, silty red-brown mudstones (Figure 13.15) with rare, thin sandstone interbeds and lenses and heterolithic units. The mudrocks are commonly bedded at a dm-scale and rarely display inverse and normal grading. These mudrocks are dominantly massive to mottled (bioturbation?), but can also display indistinct lamination, irregular horizontal and low angle cross lamination. The laminae and ripple forming sets consist of coarse-grained sand and silt. Locally, mm-thick deformed laminae occur, especially at bed boundaries, with evident injection and water-escape structures and shrinkage (synaeresis) cracks. The thin sandstone interbeds are very fine- to fine-, rarely medium-grained and occur in beds of up to 50 cm thick. They are indistinctly laminated, low-angle or flat laminated to massive in appearance, locally with cross-lamination, indistinct mm-scale argillaceous laminae, current ripples and possible wave ripples. Beds are generally sharp based and commonly contain synaeresis cracks, soft-sediment deformation (loading, sand injection, micro-faulting) and dewatering structures (fluidisation pipes). Variable degrees of bioturbation, commonly abundant, occur throughout the Formation, mainly represented by simple cylindrical burrows, and locally as dm-long vertical burrows (Skolithos, Planolites), with subordinate Teichichnus and Thalassinoides and various other rarer ichnofaunas, including undifferentiated diminutive (<1 mm across) traces and rare escape and horizontal-grazing traces. Bioturbation commonly obscures primary sedimentary structures and is locally tiered, indicating successive colonisation events and short-lived hiatus in deposition. Bioclasts (shelly fragments), pelletoidal carbonate grains and glauconite also occur. Bioclasts are only encountered in the lowermost part of the Formation. Droste (1997) reported that at the top of the Formation, just below the Barakat Formation, cuttings and well logs sometimes demonstrate the presence of dolomite occurring as thin interbeds. Cores from the eastern Ghaba Salt Basin show the presence of anhydrite lenses and nodules, chickenwire anhydrite and mud cracks.

Subsurface recognition: Lithologically there is no difference between the Mabrouk Formation shales and the shales of the overlying Barakat or the underlying Barik formations. It shows an increase in percentage of shale in the cuttings and has a negative drill break at the upper and a positive break at the lower boundary.

Post-drilling it is recognised by its monotonous, mildly serrate Gamma log with much higher maximum Gamma values than the underlying Barik Formation. The mudstones can suffer from severe washouts.

Boundaries: The Mabrouk Formation is overlain and underlain by respectively the Barakat and Barik Formations. The base is a conformable, diachronous surface based on the downwards change from redbrown claystone to clean sandstone. The top is an unconformity, generally marked by the basal Barakat sandstone. In the absence of this sandstone, the boundary is difficult to pick and relies on additional information, such as evidence for truncation, an overall log change from monotonous high Gamma shales to a cleaning-upwards trend in the Barakat Formation and the occurrence of bioclasts at the base of the Barakat Formation. Locally and at basin margins the Ghudun and the Al Khlata may directly overlie the Mabrouk Formation.

Distribution: The Mabrouk Formation is confined to North and north Central Oman, and is best developed in the Ghaba Salt Basin with a maximum thickness in the centre of the basin (>500 m). It is generally thin in well penetrations in the Fahud Salt Basin (<50 m), although seismic data suggests it may exceed 50 m in thickness to the northwest of the Fahud Salt Basin. It thins to the south and west, partially by truncation beneath the base of the Barakat Formation, but also by truncation beneath younger formations (Al Khlata Formation on the eastern edge of the Ghaba Salt Basin). The Formation also thins to the south across the Central Oman High. It is considered to be absent in South Oman either as a result of non-deposition or erosion. However, a shallow marine, bioturbated sandstone locally overlying the Mahwis Formation could be a Barakat-Mabrouk equivalent.

Deposition: The argillaceous character of the Mabrouk Formation reflects a low-energy setting where deposition was mainly from suspension settling. The rare heterolithics and sandstones represent episodic current activity, including possible wave action. The presence of bioturbation, shell fragments, marine palynomorphs and glauconite suggest a shallow-marine depositional environment, with synaeresis cracks indicating periodic fluctuations in salinity levels. The scarcity and thinness of sandstone beds suggests a distal shelfal setting, probably below storm-wave base because of the rarity of wave-formed structures. Extensive soft sediment deformation indicates rapid sedimentation and probably reflects the influence of episodic storms, with the sandstone beds thought to represent individual, periodic storm-event beds. This interpretation is supported by the occurrence of escape traces and tiering in bioturbation, reflecting largely quiescent periods interrupted by more ‘catastrophic’ events. The synaeresis cracks also suggest salinity variations that may also be related to storms.

The rapid upward decrease in sand content at the base of the Formation is interpreted as an overall deepening trend towards a major maximum flooding surface (O10 of Sharland et al., 2001). The transgressive part represents the distal portion of the retreating braid delta of the Barik Formation. Above the major flooding surface there is no major change in lithology, although a subtle cleaning-upward trend in the Gamma log may occur, which is interpreted to indicate some degree of progradation.

Droste (1997) interpreted intertidal sabkha conditions for the upper Mabrouk Formation in cores from the eastern Ghaba Salt Basin, based on the presence of anhydrite lenses and nodules, chickenwire anhydrite and mud cracks. The transition to the underlying open-marine unit is unclear (e.g. intermediate shoreface sands are missing). Droste (1997) suggested that this might indicate that the major unconformity now placed at the base of the Barakat Formation occurs within the upper Mabrouk Formation or that more erosional/onlap surfaces are present within the Mabrouk Formation. With the available data however, this cannot be proved. The presence of dolomite at the top of the Formation reported by Droste (1997) may be related to soil (calcrete) formation or to intertidal sabkha settings.

Age: Early Ordovician, Tremadocian, ca. 487–482 Ma. The Mabrouk Formation cannot be reliably distinguished from the Barakat Formation based on acritarch recovery as the lower part of the Barakat Formation contains similar assemblages to the Mabrouk Formation, which can both be attributed to the 1108B (Vulcanisphaera africana) Palyno-subzone. Consequently, both the Barakat and Mabrouk formations are placed within the Tremadocian and any absolute age assignments are both speculative and for guidance only. Sharland et al. (2001) place their early Tremadocian MFS O10 in the lower part of the Mabrouk Formation, in accordance with Droste (1997).

Biostratigraphy: The upper part of the Mabrouk Formation yields well-preserved acritarch assemblages, which can be assigned to the 1108B (Vulcanisphaera africana) Palyno-subzone (see Figure 12.1). The absence of cores and sidewall cores within this interval (and the overlying Barakat Formation) and reliance on ditch cuttings has hindered proper understanding of any changes to acritarch assemblage composition. For this reason the limit of the yielding horizons within the Mabrouk Formation is not known, but it is suspected that the lower part yields acritarchs poorly or not at all.

Barik Formation

Authors: Formation status introduced herein. The Barik Formation equates to the Barik Sandstone Member as described by Droste (1997), which was previously the sandy middle part of the ‘Andam Unit 3’ of Oprinsen (1986) and Hughes Clarke (1988).

Introduction

The Barik Formation represents a major regressive-transgressive braid delta system following the offshore deposition of the Al Bashair Formation. It comprises laterally continuous sandstones intercalated with laterally continuous mudrocks. To the north the sandstones pinch out and are replaced by a more marine silt-dominated succession. It is generally overlain by the Mabrouk Formation, occasionally by the Barakat Formation and rarely by the Al Khlata Formation.

Composite electrical logs, lithology and lithological description of the Barik Formation, Andam Group, in well Saih Nihayda-33, North Oman. See Figure 13.1 for location.

Lithology: The Barik Formation consists of thick-bedded, white-coloured, cross-bedded sandstones with intercalations of reddish mudstones (Figure 13.15), which to the north are replaced by a siltstone-dominated succession. Two discrete types of sandstone unit can be identified.

(2) Bioturbated, parallel laminated, symmetrically (wave) rippled or massive sandstones, with rare cross-bedding, that are laterally continuous. Bioturbation includes Skolithos, Diplocraterion, Planolites, and Arenicolites with other rarer forms. Thin muddy layers and drapes also occur, which in the outcrop are locally double mud drapes on ripples that indicate apparent bimodal palaeocurrents. These sandstones interbed with type (1) sandstones and amalgamate to form sand sheets, particularly to the north. In outcrop, Cruziana has been recorded and the base of some of these individual sands can contain lags with trilobite debris, and Lingula (e.g. Fortey 1994, 1995; Buckley, 1997; Droste, 1997).

The mudstones of the Barik Formation are red or red-brown, rarely pale grey, commonly bioturbated, laterally continuous and can be several metres in thickness. They contain thin sandstone and siltstone lenses and beds which are typically parallel or symmetrically (wave) ripple laminated with rare thin cross-stratified beds and possible hummocky cross-stratification. Small gutter casts occur locally. Bioturbation includes Skolithos, Diplocraterion, Planolites, Palaeophycos, Teichichnus and Arenicolites with additional rarer forms. Locally, Cruziana and undifferentiated arthropod-grazing trails occur and scattered shell debris may also be present (e.g. Millson et al., 2008). These muddy units also include synaeresis cracks and sand injection structures. Loading at the tops of these beds is common, often leading to ball and pillow structures.

Subsurface recognition: Whilst drilling, positive and negative drill breaks at upper and lower boundaries respectively can be recognised. The Barik Formation generally has a higher average Rate of Penetration than the Mabrouk or Al Bashair formations.

The Barik Formation shows the first appearance of clean sandstones below the Mabrouk or Barakat shales. In the rare instances where the Barik is overlain by the Al Khlata the formation boundary is marked by a change from the much coarser-grained sands and grey shales of the Al Khlata Formation to finer, cleaner sandstones and reddish shales of the Barik.

Post-drilling, it is characterised by the typical blocky, occasionally funnel and barrel-shaped log response with low Gamma values for the sandstones (30-60 API). The mudstones have much higher Gamma values (60–150 API), with a variable Neutron-Density separation, fluctuating from negative to positive (sand- to shale-type) separations. Mudstone interbeds are prone to washouts. Dipmeter logs typically show high angle sedimentary dips.

Boundaries: The Barik Formation is overlain by the Mabrouk, and occasionally by the Barakat. It is rarely overlain by the Al Khlata Formation, mainly along the eastern flank of the Ghaba Salt Basin. It is underlain everywhere by the Al Bashair Formation.

The lower and upper boundaries of the Barik Formation are defined lithologically (Droste, 1997). They are conformable and regionally diachronous (Millson et al., 2008).

The lower contact with mudrocks of the Al Bashair Formation is taken at the base of the lowermost major clean sandstone development or where the mudrocks become significantly silty. It reflects a change from distal marine to more proximal, storm-dominated, shelf deposits, grading up into the Barik Formation proper. The top of the unit is a diachronous transgressive surface defined by a variable, sharp to gradational contact associated with mudrocks of the overlying Mabrouk Formation and marks the return to more distal marine deposition. This boundary is well expressed on logs by a sharp shift to higher Gamma values, but the pick can be subjective if sandstones occur at the base of the Mabrouk Formation and a decision has to be taken as to which is the last ‘significant’ sandstone of the Barik Formation.

Distribution: The Barik Formation is confined to North and north Central Oman. It is absent in South Oman as a result of non-deposition or erosion. The Barik Formation trends northeast to southwest along the axis of the Ghaba Salt Basin with a maximum thickness in the centre of the basin (>400 m). It is slightly thinner, up to 400 m in the southern part of the Fahud Salt Basin. The Formation thins markedly to the north due to the depositional pinch-out of the sandstones. It is truncated to the west and in the northwestern part of the Fahud Salt Basin beneath the base Barakat unconformity and along the eastern flank of the Ghaba Salt Basin by the base Haushi unconformity. To the south it is truncated beneath both the base Barakat and base Haushi unconformities.

Mainly shoreface settings are interpreted with secondary tidal channel and mouthbar elements. In addition to these main facies, bioturbated, mudrocks accumulated in open-marine settings, but the scarcity of good wave-formed structures implies a sheltered-marine setting that was subject to repeated salinity variation (synaeresis cracks). These muddy intervals are interpreted to represent an offshore transition zone that may pass northwards into deeper water (Barik Siltstone ‘member’ - see below). Taken together these environments indicate that the Barik Formation was deposited in a northerly prograding braid delta (e.g. Droste and Mohammed, 1994; Droste, 1997; Ramseyer et al., 2004; see Figure 13.20).

The proximal part to the south is dominated by stacked, amalgamated fluvial channels, forming sheetlike reservoirs with intercalated floodplain heteroliths (e.g. Barik field) separated from one another by marine shales. Thin, marine shales even occur at the most southerly extent of the Barik Formation. The marine influence increases towards the northeast where finer grained lithologies dominate. Further north shoreface/mouthbar sandstones are dominant with considerably less fluvial deposition.

Palaeocurrent directions are typically unimodal, with the main palaeocurrent direction towards the north in both outcrop and borehole image datasets.

Subdivision: The Barik Formation can be informally divided into two lithological units which are essentially facies variations.

(1) A Barik Sandstone ‘member’ that largely corresponds to the Barik Member as described by Droste (1997); and

(2) A Barik Siltstone ‘member’ is the more distal marine, lateral equivalent of the Sandstone ‘member’ to the north.

The latter ‘member’ comprises offshore marine mudstones with thin siltstones and little or no sand. This transition also reflects the diachronous nature of both the upper and lower Barik Formation boundaries and allows for the differentiation of a silty Barik Formation from the shaly Mabrouk and Al Bashair formations.

The Barik Siltstone ‘member’ can be difficult to distinguish, especially whilst drilling, from the overlying Mabrouk and underlying Al Bashair formations as lithologies are essentially identical. A minor increase in siltstone and sandstone may be observed in cuttings. Post-drill, the unit can also be difficult to distinguish using wireline logs. It has a highly serrate Gamma profile with generally lower minimum Gamma values than are typical for the more uniformly shaly Mabrouk or Al Bashair formations and locally may have a more consistent, negative (sand-type) Neutron-Density separation.

For the Barik Sandstone, Millson et al. (2008) describe a high-resolution regional correlation framework based on wireline logs, magneto-stratigraphy, heavy mineral, core and borehole image studies. It allows a subdivision of the Barik Sandstone into lower, middle and upper units. This framework cannot currently be extended into the Siltstone ‘member’ due to the uncertainty in correlation of individual flooding events.

Age: ‘Late’ Cambrian – Early Ordovician, late Furongian – ‘early’ Tremadocian, ca. 489–487 Ma. Trilobite evidence suggests an equivalent late Furongian age for the Barik Formation at outcrop in the Al Huqf area (Fortey, 1994, 1995). The age is also constrained by palynostratigraphic data for the overlying Mabrouk Formation (Palyno-subzone 1108B, Tremadocian) and the underlying Al Bashair Formation (Palyno-subzone 1108A, Furongian) as indicated by Mohiuddin et al. (2007) and Booth (2009) (see Figure 12.1). The assumption is therefore that the interval equates to the lower part of the Tremadocian and the upper part of the Furongian, but again it must be stressed that any absolute ages are for guidance only. Interestingly, Millson et al. (2008) note up to eighteen possible magnetic polarity events of varying ‘length’ associated with the Barik Formation. Pavlov and Gallet (2001) interpret ca. 4-6 reversals per My during the ‘Middle’ Cambrian, which would be a comparable frequency to that seen in the Barik. However, they also believe that magnetic reversal frequency reduced drastically by the end of the Tremadocian.

Composite electrical logs, lithology and lithological description of the Al Bashair Formation, Andam Group, in well Saih Nihayda-33, North Oman. See Figure 13.1 for location.

Biostratigraphy: Fortey (1994, 1995) describes fragments of the late Furongian trilobite Eosaukia cf. walcotti from flat-bedded sandstones, near the base of the Barik Formation in the Al Huqf outcrop (Droste, 1997). Palynological investigation has been less successful in this interval as the succession largely comprises fluvial or oxidised marine deposits, and core and sidewall core samples have so far proved barren of organic microfossil remains.

Various non-biostratigraphic studies have been undertaken (heavy minerals, chemostratigraphy, magnetostratigraphy) to attempt to improve the stratigraphic resolution and correlation within this unit (see Millson et al., 2008).

Al Bashair Formation

Authors: Formation status introduced herein. Buckley (1997) suggested that the Al Bashair should be considered a formation, based on the outcrops in the Al Huqf area, but she retained it within the Mahatta Humaid Group. The Al Bashair Formation equates to the Al Bashair Member described by Droste (1997) and corresponds to the ‘Andam Unit 2’ (‘Limestone Member’) and lower part of the ‘Andam Unit 3’ of Oprinsen (1986) and Hughes Clarke (1988).

Introduction

Following the continental deposition of the Mahatta Humaid Group, the Al Bashair Formation marks a return to marine conditions and lies disconformably on top of the Miqrat Formation. It comprises fossiliferous mudstones with sandstone interbeds and various carbonate units. The amount and diversity of carbonates abruptly decreases in the upper half of the Formation, where red-brown mudstones dominate. This allows the division of the Al Bashair into two informal units separated by a significant maximum flooding surface (Cm30 of Sharland et al., 2001). The Al Bashair Formation is conformably overlain by the braid-delta deposits of the Barik Formation.

The upper part of the Al Bashair Formation comprises red-brown mudstones with some intercalations of sandstone beds that may contain coarse quartz grains. These deposits commonly form thin (1–2 m) coarsening-upward cycles. The base of these cycles typically consist of non-fossiliferous, red mudstones that grade to light green silty dolomitic mudstones, capped by either hummocky or swaley cross-stratified sandstones (restricted to the lowermost part of the upper Al Bashair) or bioclastic sandstones. Bed bases may show tool marks, or, more rarely, Cruziana sp. trace fossils. Pedogenic slickensides, intra-formational mud fragments and mudcracks (possible synaeresis cracks) occur within the red mudstones (Al-Marjibi, in preparation). Bioclastic sandstones contain wave ripples and are occasionally cross-bedded. Typically they tend to grade upward into bioclastic grainstones and these also only occur in the lower part of the upper Al Bashair. Bioclastic fragments comprise broken trilobite, echinoid, brachiopod and coral fragments with Lingula and associated ooids. Bioclasts are often rounded and concentrated in thin coarse-grained ‘winnow’ lags at the top of beds. The diversity and amount of bioclastic fragments significantly decreases towards the top of the Al Bashair Formation. High diversity trace fossils assemblages are common and dominated by Cruziana, Arenicolites and Diplocraterion with many additional indeterminate forms (Al-Marjibi, in preparation).

Greenish-grey siltstones and grey, very fine-grained, non-fossiliferous sandstones are interbedded on a cm-scale in the lower part of the Al Bashair Formation. These show a wide range of sedimentary structures including low angle cross-lamination and wave ripples. In outcrop the base of the Al Bashair Formation is marked by a thick (up to 4 m) bioturbated dolostone (Al-Marjibi, in preparation), with distinctive branched, horizontal trace fossils that Seilacher (1999) has interpreted to represent the action of biomat mining organisms.

The occurrence of carbonate beds consisting of oolitic grainstones, stromatolitic/oncolithic boundstones and lime-wackestones is diagnostic for the Formation. Stromatolite domes in the Al Huqf outcrop area are commonly elongated with a predominantly northeast-southwest orientation (Al-Marjibi, in preparation). Ooids also occur in some sandstone beds, with accessory bioclastic fragments (e.g. trilobite and echinoid fragments), glauconite and pyrite. These beds may display planar and herringbone crossstratification. Several levels with synaeresis cracks occur. Sandstones are commonly rich in evaporite casts (such as halite ‘hoppers’).

Subsurface recognition: Whilst drilling it can be difficult to identify the Al Bashair Formation as the dominant lithologies of the overlying Barakat, Mabrouk and Barik formations and the underlying Miqrat Formation are similar to those of the Al Bashair. However, the Al Bashair Formation is the only unit that includes carbonates. The top of the Formation is marked by a significant decrease in the amount of sandstone recorded on exiting the Barik Formation.

The Al Bashair sediments generally have a lower average Rate of Penetration than the Barik or Miqrat, but this can be variable.

Post-drilling, the moderately serrate ‘high’ Gamma log character helps to differentiate the Formation from the Barik. It has on average higher minimum API values than the overlying Barik Formation but lower ones than the underlying Miqrat Formation. The upper Gamma change is associated with a distinctive change in Neutron-Density separation. The Al Bashair is characterised by a dominantly positive (shale-type) separation and the Barik Formation alternates between positive and negative separations. Intra Al Bashair sandstones can often show similar log characteristics to sandstones of the Barik Formation, e.g. Al Bashair-1 at and below 4,765 m (Figure 13.23). However, they are distinguished and separated from the Barik sandstones by, on average, thicker intervening shale/siltstone packages.

The base of the Al Bashair Formation is more difficult to pick as there may be little difference in the log responses between the Al Bashair and Miqrat formations. However, a series of stacked coarsening-upward sequences in the lower part of the Al Bashair Formation are identifiable on the Gamma logs (e.g. Saih Rawl-32, 5,020–5,100 m, Figure 13.22).

Boundaries: The lower and upper boundaries of the Al Bashair Formation are defined by Droste (1997). The top of the Al Bashair is defined at the base of the last (downhole) ‘significant’ clean non-micaceous sandstone of the Barik Formation. This change is also seen in the shift to higher average Gamma minima in the Al Bashair. Where the Barik Formation becomes more silty to the north, this transition depends on regional correlation. The boundary is diachronous in relation to the northerly prograding braided delta system of the Barik Formation (Millson et al., 2008). The lower boundary is generally defined at the Gamma log break to higher values, seen at the top of an increasing Gamma trend in the upper Miqrat Formation. The boundary also marks the onset of the typical series of cleaning-upward cycles associated with the sands in the lower Al Bashair Formation. The lower boundary often also corresponds to the base of a thin sandstone (Droste and Mohammed, 1994). If this thin basal sandstone layer cannot be distinguished, the boundary should be put at the base of the first sandy interval above the higher Gamma mudstones of the Miqrat Formation. Droste (1997) suggested that the base Al Bashair may be a regional unconformity based on subtle onlapping geometries as seen on regional seismic lines and truncation of the underlying Miqrat and Amin formations in the Fahud Salt Basin. However, the base of the Al Bashair Formation is a marine-transgressive surface over the continental deposits of the Mahatta Humaid Group. As such, erosion at the base of the Al Bashair Formation probably represents ravinement processes that may be localised. A localised angular unconformity relating to halokinesis has also been interepreted. Vizán et al. (2009) interpret a stratigraphic gap at the base of the Al Bashair Formation in the Al Huqf outcrop area at Qarn Mahatta Humaid based on palaeomagnetic data.

Distribution: The Al Bashair Formation is confined to North and north Central Oman, but is absent in South Oman due to non-deposition or erosion. There is generally no limestone in the upper part of the Formation in west North Oman. The Al Bashair Formation trends northeast to southwest along the axis of the Ghaba Salt Basin, with a maximum thickness in the centre of the basin (>800 m). It is far thinner, up to approximately 200 m, in the Fahud Salt Basin. It is truncated to the west and in the northwestern part of the Fahud Salt Basin beneath the base Barakat unconformity and along the eastern flank of the Ghaba Salt Basin by the base Haushi unconformity and possibly also by onlap onto the margins of the basin. To the south the Formation is truncated beneath both the base Barakat and base Haushi unconformities. The decrease in thickness is most pronounced towards the south as a result of a lateral facies change into the sandstones of the Barik Sandstone ‘member’, as documented by Millson et al. (2008).

The basal part of the Al Bashair Formation is characterised by coarsening-upwards packages with an overall back-stepping trend, which indicate shallowing-upward cycles (parasequences) in a transgressive systems tract. The occurrence of halite pseudomorphs suggests a marginal marine, supratidal-intertidal environment whilst planar and herringbone cross-bedding indicate an intertidal to subtidal setting, see Droste (1997). The concentration of coarse-grains and bioclastic debris at the top of the cycles is probably the result of winnowing and may represent transgressive lags. The stromatolites and oolitic grainstones at the top of some cycles reflect a reduction in clastic input and may be associated with a relative rise in sea-level. Elongation of stromatolitic domes indicates that these have been influenced by tidal processes during growth and suggest that the palaeo-shoreline of the lower Al Bashair was orientated southeast-northwest (Al-Marjibi, in preparation). A major maximum marine flooding surface (Cm30, Sharland et al., 2001) separates the lower and upper parts of the Al Bashair Formation.

The upper part of the Formation is interpreted as a prograding unit of shallow-marine sediments in front of the braided delta system of the Barik Formation. Bioclastic fragments, diverse trace fossil assemblages and wave ripples suggest deposition in a shallow-marine setting. Other features (e.g. pedogenic slickensides, intra-formational mudcracks, synaeresis/desiccation cracks) indicate deposition in very shallow subaqueous settings with periods of subaerial exposure. These could be either playa deposits in a supratidal setting or a very shallow and restricted marine setting (e.g. lagoon). Within this framework, the occurrence of hummocky and swaley cross-stratification, particularly towards the base, suggests the periodic occurrence of intense currents associated with storm to storm-surge and ebb current activity. The diversity and amount of the bioclastic fragments and bioturbation decreases towards the top of the Al Bashair Formation, which reflects a diminishing marine influence and may indicate more restricted conditions (lower salinity). This probably reflects the approach of the Barik Formation braid delta system. Evidence for fully open-marine conditions has not been encountered in the Al Bashair Formation.

Subdivision:Droste (1997) applied a subdivision of the Al Bashair into a lower and upper part, separated by a maximum flooding surface (see his figures 11 and 12). This flooding is MFS Cm30 of Sharland et al. (2001). The lower part is characterised by the occurrence of dolomite/limestone beds and by shallowing-upwards, backstepping parasequences. The upper part is characterised by thin (1–2 m) coarsening-upwards successions and rare limestones, with subtle cleaning-upwards or straight Gamma ray motifs. The boundary between the two is placed at the aforementioned maximum flooding surface, which is marked by a Gamma ray peak that corresponds to the top of the more limestone-rich succession. The maximum flooding surface approximates to a seismic horizon, informally known as the ‘intra-Al Bashair marker’, which can be mapped across much of North Oman.

Age: ‘Late’ Cambrian, Furongian, ca. 499–489 Ma. Fortey (1994, 1995) interpreted a ‘Late’ Cambrian, Furongian-equivalent age, based on trilobites found in the Al Huqf outcrops. This is supported by palynological studies from well sections, which indicate Palyno-subzone 1108A (Veryhachium? dumontii). Sharland et al. (2001) place their mid-‘Late’ Cambrian MFS Cm30 in the middle of the Formation, in accordance with MFS1 of Droste (1997).

Biostratigraphy: Bioclastic fragments from primitive corals, shell fragments, various burrow types, shells of Lingula type, trilobite debris and Cruziana sp. have been observed in outcrop. A distinctive acritarch assemblage can often be obtained from the middle and most marine part of the Al Bashair Formation, which equates to MFS Cm30 of Sharland et al. (2001). The palynomorphs are assigned to Palyno-subzone 1108A (Veryhachium? dumontii) which implies a Furongian age (Mohiuddin et al., 2007; Booth, 2009, see Figure 12.1).

Trilobite assemblages, which include Paracoosia cf. mirzadi and Changshania kürsteni were described from the Al Bashair Formation Al Huqf outcrops by Fortey (1994, 1995). These suggest an equivalent early to middle Furongian age and therefore support the palynological evidence.

1

The Cambrian timescale is currently in a state of flux with four global series and ten international stages (Ogg et al., 2008), therefore ‘Late’, ‘Middle’ and ‘Early’ designations are used informally.